1. Field of the Invention
This invention relates to a plasma display panel formed by sealing up the peripheries of a pair of opposed substrates via discharge spaces held between the substrates with a sealing material and a method of producing the same.
2. Background Art
A plasma display panel (PDP) is formed by partitioning an airtight space between a pair of substrates into a plurality of discharge spaces in stripes or matrices, so that an image is displayed by selectively generating electric discharge in the plurality of discharge spaces.
FIG. 3 is a diagram illustrating the general structure of a plasma display panel, wherein reference numeral 1 denotes a front glass substrate having a display electrode 2, a dielectric layer 3, a protective layer 4 of such as MgO (magnesium oxide) and so on. Reference numeral 5 denotes a back glass substrate on which partition walls 6 are formed at predetermined intervals on a dielectric layer 9. The space formed between the front glass substrate 1 and the back glass substrate 5 is partitioned by the partition walls 6 into minute spaces which serve as discharge spaces. Address electrodes 7 and phosphor layers 8a, 8b and 8c for RGB colors are formed in the respective discharge spaces and further a neon-xenon mixed gas, for example, is also injected into the discharge spaces.
In the case of a plasma display panel like this, an image is displayed by applying voltage across the display electrode 2 and the address electrodes 7 so as to selectively cause each of the phosphor layers 8a, 8b and 8c formed on the inner surface of the discharge spaces to discharge and to emit light therefrom. In this case, a black stripe 10 corresponding to the surface of each partition wall 6 is formed on the back of the front glass substrate 1. In the example shown in FIG. 3, though the partition walls 6 are provided in the form of a well curb so as to arrange discharge spaces in matrices, there is also a method of providing discharge spaces in stripes by forming the partition walls 6 in parallel to each other.
A method of producing such a plasma display panel will be described. A front and a back panel are formed through the steps of successively forming the front-panel component parts including the display electrode 2, the dielectric layer 3 and the protective layer 4 on one side of the front glass substrate 1 and successively forming the back-panel component parts including the address electrodes 7, the partition walls 6 and the phosphor layers 8a, 8b and 8c on one side of the back glass substrate 5, the front and back panels being stacked up so that the panel component parts face one another. Further, the peripheral portions of the front glass substrate 1 and the back glass substrate 5 are sealed up with a sealing material and internal evacuation of the discharge spaces formed between the two sheets of substrates thus bonded together is carried out. Then a discharge gas is encapsulated in the discharge spaces and the whole structure is subjected to aging.
At the sealing step above, a sealing material made of fritted glass having a low melting point is applied by screen printing, in a frame-like figure, to the peripheral portion of the front glass substrate 1 or back glass substrate 5, or the peripheral portions of both the glass substrates 1 and 5. Then the sealing material is temporarily calcined to form a sealing layer and both the glass substrates 1 and 5 are stacked and subjected to heat treatment at about 400° C. while both the substrates are pressed against each other, whereby the space between both the glass substrates is sealed up as the sealing layer is softened and fusion-bonded.
During the process of producing the plasma display panel above, as pressure fluctuations resulting from the internal evacuation of the discharge spaces and the encapsulation of the discharge gas in the discharge spaces are caused to the structure of the panel in such a condition that the sufficient strength of the softened and fusion-bonded sealing layer remains unacquired after the heat treatment is made at the sealing step, there occurs a phenomenon of causing the end portions on the peripheries of the glass substrates to become curved, whereas in the substantially central portions of the glass substrates, there occurs a phenomenon of increasing the substrate-to-substrate space due to the pressure of encapsulated discharge gas.
FIG. 4 shows the above situation byway of example, wherein as a sealing material 20 contracts, the dimension of the space between the opposed glass substrates 1 and 5 becomes uneven in the substantially central portions of the glass substrates and this results in narrowing the space between the peripheral portions thereof. In the plasma display panel, though a uniform space has to be set between the glass substrates 1 and 5 so as to obtained desired characteristics, the problem is that the desired display characteristics remain unobtainable in the situation shown in FIG. 4.
In such a situation that the glass substrates 1 and 5 are warped when the sealing material 20 contracts as shown in FIG. 4, the curving of the glass substrates causes the display quality to lower and there is produced a gap G between the partition walls 6 and the front panel and the problem in this case is that an abnormal sound of about 10 kHz is generated because that gap portion vibrates when the panel is driven.
Heretofore, a glass or plastic substrate 30 has been arranged as a sound insulating member over the front glass substrate 1 to counter the generation of the abnormal sound. Further, an optical film 31 serving as an electromagnetic-wave cutting off layer for cutting off an electromagnetic wave leaking out from a plasma display panel, an infrared-ray absorbing layer for absorbing an infrared ray emitted from the plasma display panel or an external-light reflection preventive layer for preventing the external light from being reflected have been bonded onto the substrate 30. However, the provision of the substrate 30 has raised the problem of not only increasing the number of parts but also impeding an attempt to reduce the thickness of the panel.